1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a lighting system with color compensation for electronic light sources that responds to changing dim levels and changing temperature.
2. Description of the Related Art
Electronic light sources, such as light emitting diodes (LEDs), offer lower energy consumption and, in some instances, longer useful life relative to incandescent bulbs. In some instances, lamps with LEDs are designed to approximate the familiar color characteristics of incandescent bulbs. LEDs with different color spectra can be mixed within a lamp to approximate the color of an incandescent bulb. The color spectrum (e.g. the dominant wavelength) and brightness (i.e. luminosity) of an LED is a function of the junction temperature of the LED. Thus, as the junction temperature changes, the color of the LEDs can also change. The color spectrum of some LEDs varies with the junction temperatures of the LEDs more than others. For example, the brightness of blue-white LEDs varies less with temperature than that of red-amber LEDs. When the brightness from a mix of multi-colored LEDs changes, especially, when the brightness of one color changes more with respect to another color, the changing brightness causes the perceived color of the mix of the LEDs to change. Thus, to maintain a constant color of a group of LEDs, circuits have been developed to maintain a constant color as the junction temperature changes by adjusting the currents to counteract the changes induced by temperature.
The color of a light source, such as an LED, is often referenced as a “correlated color temperature” (CCT) or as a “color spectrum”. The CCT of a light source is the temperature of an ideal black-body radiator that radiates light that is perceived as the same color as the light source. The color spectrum of a light source refers to the distribution of wavelengths of light emitted by the light source. Both CCT and color spectrum represent characteristics to classify the color of a light source.
FIG. 1 depicts a lighting system 100 that includes a lamp 101 that includes a lamp 101, and the lamp 101 includes two sets of LEDs referred to as LEDs 102 and LEDs 104. LEDs 102 have a red-amber color spectrum, and LEDs 104 have a blue-white color spectrum. The overall spectrum of the light from lamp 101 is a mixture of the color spectra from LEDs 102 and LEDs 104 and varies with the intensity (i.e. brightness) of the respective LEDs 102 and LEDs 104. The intensity of LEDs 102 and LEDs 104 is a function of the respective currents iLED—A and iLED—B to LEDs 102 and LEDs 104.
The lighting system 100 receives an AC supply voltage VIN from voltage supply 106. The supply voltage VSUPPLY is, for example, a nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe and the People's Republic of China. The full-bridge diode rectifier 105 rectifies the supply voltage VSUPPLY for input to switching power converter 110. Controller 112 controls the switching power converter 110 to generate a light source current iLS. Capacitors 120 and 122 each provide a standard filter across respective LEDs 102 and LEDs 104.
The current distributor 114 controls the current dividers 116 and 118 to respectively apportion the light source current iLS as iLED—A to LEDs 102 and iLED—B to LEDs 104. Since the proportional intensity of LEDs 102 and LEDs 104 and, thus, the color spectrum of lamp 101, is a function of the currents iLED—A and iLED—B, by apportioning the current distributed to LEDs 102 and 104, the current distributor 114 causes the lamp 101 to generate a proportion of red-amber color to white-blue color to approximate the color spectra of an incandescent bulb.
The lamp 101 includes a negative temperature coefficient (NTC) resistor 117 to allow the current distributor 114 to sense the ambient temperature in proximity to LEDS 102 and LEDs 104. The resistance of NTC resistor 117 is indirectly proportional to changes in the ambient temperature. Changes in the value of TDATA associated with changes in the resistance of the NTC resistor 117 represent changes in the ambient temperature. Thus, by determining the value of TDATA, the current distributor 114 senses changes in the ambient temperature in proximity to LEDs 102 and LEDs 104.
The spectrum of red-amber LEDs 102 is more sensitive to junction temperature changes than the blue-white LEDs 104. As the ambient temperature in proximity to LEDs 102 and LEDs 104 changes, the junction temperatures also change. Sensing the ambient temperature in proximity to LEDs 102 and LEDs 104 represents an indirect mechanism for sensing changes in the junction temperatures of LEDs 102 and LEDs 104. Thus, sensing the ambient temperature approximates sensing the respective color spectrum of LEDS 102 and LEDs 104. Accordingly, as the ambient temperature changes, the current distributor 114 adjusts the currents iLED—A and iLED—B to maintain an approximately constant color spectrum of lamp 101.
However, the lighting system 100 relies on analog components to maintain the approximately constant color spectrum of lamp 101. Analog components are subject to variations due to temperature and fabrication tolerances and tend to limit the accuracy of the system. Furthermore, many lighting systems include dimmers to dim lamps. The dimmers set a particular dim level by, for example, modulating a phase angle of a supply voltage. It would be desirable to dynamically respond to changes in both the dim level and temperature in a multi-LED lighting system.